1. Field of the Invention
This invention relates to an improved process for oil reclamation by gas injection into oil-bearing formations in which process the gas is produced by a wet oxidation reaction.
2. Description of the Prior Art
Conventional methods of recovering crude oil from underground reservoirs succeed in producing only about 30 percent of the total oil in the underground formation. The term "Enhanced Oil Recovery" (EOR) refers to techniques that are in use or have been proposed for the purpose of recovering all, or a portion of, the 70 percent of the oil remaining in these formations. In addition, some of the heavier (more viscous) crude oils cannot be produced at all without the use of EOR. For a detailed description of the prior art see "Enhanced Oil-Recovery Techniques--State-of-the-Art Review", by N. Gangoli and G. Thodos, Journal of Canadian Petroleum Technology, pp. 13-20 (October-December 1977).
The EOR processes include vapor or gas injection methods of which the following are exemplary:
(a) Steam Flooding
Steam is injected into a reservoir and oil is produced at an adjacent well (steam drive) or, at a later time, from the same well that is used for injection (steam soak or "huff and puff"). The steam heats the oil and reduces its viscosity so that it can flow to the production well; for example, see Bergstrom U.S. Pat. No. 3,057,404 (Oct. 9, 1962), and Schlinger U.S. Pat. No. 4,007,786 (Feb. 15, 1977).
(b) Carbon Dioxide Miscible Flooding
Carbon dioxide is injected into a reservoir and oil is produced from an adjacent well. The carbon dioxide dissolves in the oil and the viscosity of the mixture is significantly reduced compared to that of the native oil; for example, see Keith U.S. Pat. No. 3,442,332 (May 6, 1969), Brown U.S. Pat. No. 3,871,451 (Mar. 18, 1975), and "Carbon Dioxide Miscible Flooding: Past, Present, and Outlook for the Future" by F. I. Stalkup, Journal of Petroleum Technology, pp. 1102-1112 (August 1978).
(c) Gas Pressurization
Inert gas ("flue gas", "exhaust gas", nitrogen, etc.) is injected into a reservoir and oil is produced from an adjacent well. Gas pressure drives the oil toward the production well; for example see "Enhanced-recovery inert gas processes compared", by K. Wilson, The Oil and Gas Journal, pp. 162-166, 171-2 (July 31, 1978).
It has also become evident that combinations of the above techniques, i.e. mixtures of inert gas, nitrogen (N.sub.2), carbon dioxide (CO.sub.2) and water vapor (steam), can have significant benefits for EOR, particularly for heavy oils; for example, see West et al. U.S. Pat. No. 3,782,470 (Jan. 1, 1974) and Sperry et al. U.S. Pat. No. 3,948,323 (Apr. 6, 1976).
Different mixtures of N.sub.2, CO.sub.2 and steam will have different effects on oil recovery, and for a given oil reservoir a particular composition will optimize oil recovery. For example, it is possible to produce steam by means of a conventional boiler and then blend in with the steam either compressed flue gas or carbon dioxide. It is also possible to burn fuel in a high pressure combustor and inject water into the hot gas stream generated thereby, as taught by Sperry et al. U.S. Pat. Nos. 3,948,323 and 3,993,135, and Walter U.S. Pat. No. 2,734,578.
There are problems with these techniques in the prior art. Steam generators for EOR, often called "oil field steam flooders", must burn expensive and scarce fuels such as natural gas, refined petroleum products, or in some cases, the oil that is produced by the EOR technique itself. Burning even clean fuels, but especially sulphur containing produced oil, generates air pollution problems. Feedwater for oil field flooders must be 100 percent made up, since there is no condensate return. The condensed water produced along with the oil must be treated before disposal. In addition to inorganic contaminants, this produced water contains residual oil which is uneconomical to recover by present technology. It has been proposed to use the produced water as feedwater for the steam flooders, but this requires even more elaborate and expensive treatment, including deoiling, softening to a hardness level below 5 ppm, and silica reduction. Several water treatment schemes have been proposed by M. J. Whalley and T. M. Wilson, Water Conservation in a Steam Stimulation Project, First International Conference on the Future of Heavy Crude and Tar Sands, Edmonton, Alberta, June 8, 1979.
Nearly pure CO.sub.2 can be obtained from natural reservoirs or from certain manufacturing processes. Such CO.sub.2 must be dried, compressed and transported by pipe line to the point of use for EOR. However these sources of CO.sub.2 are limited in quantity and cannot supply the predicted demand. CO.sub.2 can be generated by burning fuel in a conventional boiler, absorbing CO.sub.2 from the flue gas with certain organic solvents, stripping the CO.sub.2 from the solvent, and compressing the CO.sub.2 for use. It has been reported that as much as one-half of the energy produced by burning the fuel for this process must be used for stripping the CO.sub.2 from the solvent. Oxides of nitrogen are produced and must be removed from the gas stream. In any case, the produced water must be treated and disposed of.
Inert gas can also be generated by burning clean fuel. The combustion must be carefully controlled so as to minimize residual oxygen and oxides of nitrogen. Since the gas must be compressed after combustion, careful treatment is required to eliminate corrosion and fouling in the compressor.
Many of the disadvantages of the prior art processes are avoided or minimized by the present invention which employs wet oxidation as a source of the injection gas. Wet oxidation is a term used for a self-sustained oxidation of any combustible material, including low grade fuels, organic waste materials, and reduced forms of inorganic materials, in aqueous medium, initiated at elevated temperatures and pressures. The oxidizing agent can be pure oxygen, air or mixtures thereof. The gaseous effluent of the wet oxidation is comprised essentially of water vapor, carbon dioxide and nitrogen (if air is used), although small amounts of carbon monoxide, residual oxygen and volatile organic compounds may be present. Illustrative of prior art wet oxidation processes are those disclosed in Zimmermann U.S. Pat. No. 2,824,058 (Feb. 18, 1958) and Pradt U.S. Pat. No. 4,100,730 (July 18, 1978).
3. Prior Publication
Certain aspects of the instant invention have been described in a manuscript of a paper presented at the First International Conference on the Future of Heavy Crude and Tar Sands in Edmonton, Alberta on June 7, 1979, author Z. G. Havlena. This publication was made subsequent to the time the invention disclosed and claimed herein was made, and the pertinent disclosure of said publication was derived from the inventors of the instant invention.